Such ballasts are known per se. They frequently have half-bridge inverter circuits. However, the invention also relates to other ballasts. In principle, an inverter circuit generates from a rectified AC voltage supply or a DC voltage supply a supply power for the lamp which has a higher frequency than the system frequency. In many cases, a control circuit is provided here for controlling the lamp current or the lamp power during continuous operation of the lamp, and this will be referred to below as the continuous-operation control circuit. This continuous-operation control circuit influences the operating frequency at which the inverter supplies power to the lamp and thereby controls the lamp current or the lamp power. This takes place by bringing the operating frequency closer to or further away from resonant frequencies of lamp resonant circuits containing the lamp.
Before the lamp can be operated, it has to be started by a relatively high voltage. For this purpose too, resonance excitation of the lamp resonant circuit is used in many cases. In the case of discharge lamps having preheatable electrodes, the electrodes are initially preheated for a specific time before the actual starting voltage is applied. The preheating time is in this case determined by a preheating timer, in which, in the most general sense, a physical operation runs which defines a temporal delay, and, once the preheating time has expired, must return in order to be able to run again for subsequently switching the lamp on again. The preheating timer in this case has the function of a switch. The details on the implementation of such a preheating timer and the physical operation are not relevant to the principle of the invention, for which reason the abovementioned general wording has been selected.
In such cases, starting of the lamp takes place independently of the continuous-operation control circuit once said physical operation has run. For this purpose, in some way the starting voltage must be reached, for example by resonance excitation in the lamp resonant circuit. In this case the influence of the continuous-operation control circuit would have a disruptive effect.